Due to their high biocompatibility, calcium phosphate type ceramics have conventionally been used as biomaterials such as artificial tooth roots and bones. Because the crystal particles (primary particles) of these ceramics are unoriented and these ceramics are unoriented polycrystalline bodies, the mechanical properties of these ceramics are isotropic. When these ceramics are placed under stress, the propagation of a fracture will travel over the shortest distance irrespective of the direction in which the stress is applied and the surface energy for a fracture becomes so small that it will eventually cause a reduction in the fracture toughness of a shaped article of these ceramics and hence sinters thereof. Attempts have been made to increase the strength of these ceramics by sintering them for high density after performing a suitable treatment such as HIP (hot isotactic press), but no success has been achieved in improving their fracture toughness (as described, e.g., in Preprints of Annual Meeting of the Ceramics Society of Japan 1984, 3G10, pp. 939, published on May 14, 1984).
Hydroxyapatite which is in the class of calcium phosphate type compounds has ionic conductivity and research has been undertaken to study its use as an electronic material in devices such as humidity sensors. However, conventional hydroxyapatite ceramics are also isotropic with respect to electrical properties, and conduction ions in the ceramics are diffused at grain boundaries, making it impossible to provide satisfactory conductivity to the ceramics (As described, e.g., in Preprints of 2nd Apatite Meeting, pp. 22, published on Dec. 1, 1986).
In order to exploit the above various properties of calcium phosphate type ceramics in an advantageous way, it is necessary that shaped articles or sinters thereof have anisotropy. The ideal method for meeting this need would be to use single crystals of these ceramics. However, it is generally difficult to prepare large single crystals of calcium phosphate, and, in fact, no report has been published that describes success in preparing large single crystals of calcium phosphate.
In order to attain anisotropy for various properties of shaped articles and sinters, it is necessary that at least two crystallographic axes or at least two crystal faces each must be oriented in one direction, respectively. For example, in tubular bones of animals, the c axis of apatite crystals in the bone is oriented in the direction parallel to the longitudinal direction of the bone, and the a axis thereof is oriented in the direction perpendicular to the longitudinal direction of the bone, by which the anisotropy of the mechanical properties is attained.
Hydroxyapatite sinters in which one crystal face of the crystalline particles is oriented by hot-pressing has been reported in Preprints of Annual Meeting of the Ceramic Society of Japan 1984, A-72, pp. 511, published on May 14, 1984. In this method, because tabular (plate-like) crystals are used, the (h00) plane is oriented in the pressing direction, but there is no research whether other planes (such as the (00l) plane) are oriented in one direction. Therefore, it is unknown whether sinters having anisotropy for mechanical properties are obtained by this method.